In the oil industry, testing of wettability of oil reservoir in rocks is the basis for testing oil physical property, and wettability of tight oil reservoir affects oil-water microcosmic distribution, bound water saturation, residual oil saturation, capillary force and relative permeability and etc. of the reservoir. Accurate testing of the wettability of the tight oil reservoir can help optimization of site development, which has an important guiding role on effective exploitation of the tight oil reservoir.
Currently in the prior art, methods for testing wettability of tight oil reservoir mainly include a contact angle method and a self-absorption method.
The contact angle method mainly includes testing a contact angle of oil and water to a surface of the oil reservoir, estimating an oil-wetting degree and a water-wetting degree of the tight oil reservoir, and then obtaining the wettability of the tight oil reservoir, but this method can only test the wettability of a partial oil reservoir in rocks. As for the tight oil reservoir, the wettability of the partial oil reservoir cannot represent the wettability of the whole oil reservoir.
The self-absorption method is the most widely used method for testing the wettability of the tight oil reservoir in the current, and this method mainly includes measuring and comparing an amount of self-absorbed oil (or self-absorbed water) by a capillary tube and a water displacement oil discharge volume (or an oil displacement water discharge volume) when the oil reservoir is in a residual oil (or bound water) state, and performing a corresponding calculation to obtain the wettability of the oil reservoir. This method can test quantitatively the wettability of the overall oil reservoir. But as for tight oil reservoir, since there are a large amount of micron-sized or even nano-sized air voids in the tight oil reservoir, self-absorption process takes a very long time, usually more than 15 days, and meanwhile it is very difficult to ensure a sufficient self-absorption. In addition, since the air voids in the tight oil reservoir are extremely small, the tight oil reservoir can accommodate an extremely small amount of fluid, so that when the amount of the fluid discharged by self-absorption is measured by a metering tube, it is inevitable to produce a large error, causing a reduction in accuracy of a test result.
The existing techniques contain at least the following defect: the wettability of the overall oil reservoir cannot be tested by the contact angle method. Testing the wettability of tight oil reservoir by the self-absorption method will take a long time, has a low testing efficiency, and meanwhile may produce an inevitable system error, and thus the testing accuracy is low. In addition, the self-absorption method cannot analyze quantitatively the water-wetting degree and the oil-wetting degree of the tight oil reservoir.